This invention relates to flowmeter calibration and, more particularly, to a method for determining the K-factor of a flowmeter with a prover or calibrator.
In order to obtain accurate readings from a flowmeter, it must be calibrated from time to time by determining its characteristic, i.e., the constant of proportionality between the flow rate of the fluid flowing through the flowmeter and the response given by the flowmeter, sometimes called the K-factor of the flowmeter. In the case of a turbine type flowmeter that develops electrical oscillations proportional in number to the volume of flow through the flowmeter, the K-factor is expressed in terms of the number of pulses generated by the flowmeter per unit volume of fluid passing therethrough. Apparatus to determine the K-factor with the flowmeter in an operating fluid system is called a prover. Apparatus to determine the K-factor with the flowmeter in a self-contained testing system, i.e., not in an operating fluid system, is called a calibrator.
My U.S. Pat. No. 4,152,922 discloses a small-volume prover that employs mechanical volume displacement techniques. The prover has a measuring piston that travels through a measuring cylinder as a fluid barrier in synchronism with fluid passing through the operating fluid system that includes the flowmeter under test. A rod connects the measuring piston to a fluidically actuated control piston in a control cylinder which serves to hold the measuring piston at the upstream end of the measuring cylinder between test runs and return the measuring piston to the upstream end of the measuring cylinder after each test run.
The K-factor of a flowmeter varies as a function of the flow rate and the fluid characteristics such as viscosity, density, temperature and pressure. Thus, many test runs of the calibrator or prover must be made to derive the K-factor as a function of all the variables that the flowmeter may encounter. As a result of this consideration, speed and automation of the calibrator or prover operation are important factors. Another important factor is the capacity of the calibrator or prover, i.e., the fluid volume it can displace during a test run. The higher the capacity, the more accurate and repeatable is the determination of the K-factor. On the other hand, the cost of manufacture and space requirements go up as the capacity increases.
Small volume calibrators and provers generally use one of two techniques to generate K-factor data. The one technique, now commonly called dual chronometry, is described in Francisco U.S. Pat. No. 3,403,544, which issued on Oct. 1, 1968. A fixed volume of fluid is displaced by the measuring piston of the calibrator or prover during data acquisition in each test run. This fixed volume is determined by the placement of two piston sensors to measure the time interval between which the measuring piston travels from an upstream position to a downstream position. The sensors must be spaced sufficiently far from the upstream end of the cylinder through which the measuring piston travels so the measuring piston is up to full speed as it passes the upstream sensor. The sensors must also be spaced apart sufficiently to average out steady state flow perturbations. The apparatus cannot easily handle a wide range of flow rates and fluid characteristics without repositioning the piston sensors, which requires the performance of a time consuming water draw operation.
The other technique permits the volume of fluid displaced during data acquisition in each test run to be varied according to the flow rate and fluid characteristics. As disclosed in Francisco U.S. Pat. No. 3,492,856, which issued Feb. 3, 1970, an encoder generates a signal representative of the piston position throughout measuring cylinder travel with high resolution. The encoder can be a rotary electrical encoder or a linear optical encoder. The piston position at the start of data acquisition and the displaced volume during data acquisition can be selected to provide the most accurate and reliable K-factor data, depending upon the flow rate and fluid characteristics. This selection seeks to make the best compromise of the following countervailing considerations: the further downstream the piston is, when data acquisition begins, the more assurance there is that the measuring piston is up to full speed, but the less opportunity there is to average out steady state perturbations in the flow rate during data acquisition.